In general, a particle beam therapy system is provided with a beam generation apparatus that generates a charged particle beam, an accelerator that is connected with the beam generation apparatus and accelerates a generated charged particle beam, a charged particle beam transport system that transports a charged particle beam that is accelerated by the accelerator so as to gain predetermined energy and then is emitted, and a particle beam irradiation apparatus, disposed at the downstream side of the beam transport system, for irradiating a charged particle beam onto an irradiation subject.
In general, the motion of a charged particle in a charged particle beam in a charged particle beam transport system is described based on a coordinate system in which s direction is the traveling direction of a beam, x direction is a direction that is perpendicular to s direction and is the deflection direction of an accelerator, and y direction is a direction perpendicular to both direction and x direction. In this situation, the distribution of a charged particle in a beam in the charged particle beam transport system is not uniform. In general, in the case of a slow-extraction method in which a beam is slowly extracted from the synchrotron, which is an accelerator, the y-direction distribution of a charged particle is a Gaussian distribution, and the x-direction distribution thereof is a non-Gaussian distribution; thus, the respective emittances corresponding to the x-direction and y-direction areas in the phase space of the beam are asymmetric with each other. The slow-extraction method denotes a method in which charged particle beams are extracted little by little from a synchrotron over a long period.
When attention is focused on the motions of charged particle, the respective traveling directions thereof are different from each other and change with time; the beams, as a whole, perform a constant-period oscillation that is called a betatron oscillation. The angle and the width of the beam ellipse of a charged particle beam in a phase space can be changed by a quadrupole electromagnet or the like; however, even in this case, the emittance (the area of the ellipse) is kept constant. In general, in a charged particle beam transport system, the x-direction and y-direction emittances are asymmetric with each other, and this difference cannot be cancelled even by the quadrupole electromagnet or the like.
As described above, the emittances are asymmetric with each other; therefore, when the treatment plan for a particle beam therapy is made, it is difficult to secure the uniformity of a dose irradiated onto a diseased site. That is to say, the fact that the distribution of x-direction charged particles on a diseased site is a non-Gaussian distribution makes treatment planning complicated and makes an actually irradiated dose non-uniform, although the non-uniformity is within a tolerance range. When it is required to reduce the exposure amount of normal tissues as much as possible while a particle beam is intensively irradiated onto a diseased site, it is preferable to utilize a rotating gantry for rotating a particle beam irradiation apparatus around the patient. However, when there exists the non-uniformity of emittance, the spot shape of a beam on the diseased site changes depending on the rotation angle of the rotating gantry, thereby making treatment planning difficult.
Patent Document 1 discloses a charged particle beam transport system that is provided with an emittance adjustment means such as a skew quadrupole electromagnet or a solenoid electromagnet, for the purpose of symmetrizing the emittances. Patent Document 2 discloses a charged particle beam irradiation apparatus that is provided with a scatterer and a downstream electromagnet formed of a plurality of quadrupole electromagnets provided at the downstream side of the scatterer in a charged particle beam transport system. The scatterer and the downstream electromagnet symmetrize the emittances and make each of the x-direction and y-direction distributions of charged particles become a Gaussian distribution.